A polarizing film is an optical member having a function to transmit specific linearly polarized light from polarized light or natural light.
General-purpose polarizing films are obtained by, for example, drawing a polyvinyl alcohol film dyed with iodine.
Polarizing films obtained by a solution casting method are also known. The solution casting method is a method of forming a polarizing film on a substrate by applying onto the substrate a coating solution containing a colorant material and a solvent.
A polarizing film obtained by a solution coating method has the advantage that its thickness is remarkably small as compared to a polarizing film obtained by drawing the polyvinyl alcohol film as described above.
Conventionally, a polarizing film containing a disazo compound represented by the following general formula (I) is known as a polarizing film obtained by a solution coating method (Patent Document 1).

In the general formula (I), A represents a naphthyl group that may be substituted with a hydroxyl group, an amino group, or a sulfonic acid group, and B represents a phenylene group or a naphthylene group that may be substituted with a lower alkyl group, a lower alkoxy group, a hydroxyl group, or a sulfonic acid group.
However, the polarizing film in Patent Document 1 has the problem of poor transparency and a low dichroic ratio.
Further, a disazo compound that can be inexpensively synthesized is required for producing an inexpensive polarizing film.
[Patent Document 1]
Japanese Examined Patent Publication No. 7-92531 (Japanese Patent Laid-open Publication No. 62-330213)
An object of the present invention is to provide a polarizing film having a high dichroic ratio and a method for producing the polarizing film.
The polarizing film of the present invention contains a disazo compound represented by the following general formula (1).

Q1 represents a phenyl group having at least one —SO3M group, Q2 represents a naphthylene group having at least one —SO3M group, X represents an alkyl group having 1 to 4 carbon atoms, an alkoxy group having 1 to 4 carbon atoms, a thioalkyl group having 1 to 4 carbon atoms, a halogeno group, or a polar group, subscript k denotes the number of substitutions of X and represents an integer of 0 to 5, and M represents a counter ion, where when k is an integer of 2 or more, Xs are the same or different.
Preferably, Q2 is a naphthylene group represented by the following general formula (Q2-1), and more preferably it is a naphthylene group represented by any one of the following formulae.

Z represents a substituent other than the —SO3M group, subscript o denotes the number of substitutions of Z and represents an integer of 0 to 5, and subscript n denotes the number of substitutions of the —SO3M group and represents an integer of 1 to 6, where 1≦n+o≦6, and when o is 2 or more. Zs are the same or different.

Preferably, Q1 is a phenyl group represented by the following general formula (Q1-1), and more preferably a phenyl group represented by any one of the following formulae.

Y represents a substituent other than the —SO3M group, subscript m denotes the number of substitutions of Y and represents an integer of 0 to 4, and subscript l denotes the number of substitutions of the —SO3M group and represents an integer of 1 to 5, where 1≦l+m≦5, and when m is 2 or more, Ys are the same or different.

In another aspect of the present invention, an image display device is provided.
This image display device includes any one of the polarizing film described above as its composition member.
In another aspect of the present invention, a method for producing a polarizing film is provided.
This method for producing a polarizing film includes a step of applying onto a substrate a coating solution containing a disazo compound represented by the above general formula (1) and a solvent.
A polarizing film of the present invention contains a disazo compound represented by the general formula (1), and therefore has a high dichroic ratio.
An image display device including such a polarizing film having a high dichroic ratio is excellent in display characteristics.